1. Field of the Invention
The present invention relates generally to cellulose esters and/or ionic liquids. One aspect of the invention concerns processes for producing cellulose esters in ionic liquids.
2. Description of the Related Art
Cellulose is a β-1,4-linked polymer of anhydroglucose. Cellulose is typically a high molecular weight, polydisperse polymer that is insoluble in water and virtually all common organic solvents. The use of unmodified cellulose from wood or cotton products, such as in the housing or fabric industries, is well known. Unmodified cellulose is also utilized in a variety of other applications usually as a film (e.g., cellophane), as a fiber (e.g., viscose rayon), or as a powder (e.g., microcrystalline cellulose) used in pharmaceutical applications. Modified cellulose, including cellulose esters, are also utilized in a wide variety of commercial applications. Cellulose esters can generally be prepared by first converting cellulose to a cellulose triester, then hydrolyzing the cellulose triester in an acidic aqueous media to the desired degree of substitution (“DS”), which is the average number of ester substituents per anhydroglucose monomer. Hydrolysis of cellulose triesters containing a single type of acyl substituent under these conditions can yield a random copolymer that can comprise up to 8 different monomers depending upon the final DS.
Ionic liquids (“ILs”) are liquids containing substantially only anions and cations. Room temperature ionic liquids (“RTILs”) are ionic liquids that are in liquid form at standard temperature and pressure. The cations associated with ILs are structurally diverse, but generally contain one or more nitrogens that are part of a ring structure and can be converted to a quaternary ammonium. Examples of these cations include pyridinum, pyridazinium, pyrimidinium, pyrazinium, imidazolium, pyrazolium, oxazolium, triazolium, thiazolium, piperidinium, pyrrolidinium, quinolinium, and isoquinolinium. The anions associated with ILs can also be structurally diverse and can have a significant impact on the solubility of the ILs in different media. For example, ILs containing hydrophobic anions such as hexafluorophosphates or triflimides have very low solubilities in water, while ILs containing hydrophilic anions such chloride or acetate are completely miscible in water.
The names of ionic liquids can generally be abbreviated according to the following convention. Alkyl cations are often named by the first letters of the alkyl substituents and the cation, which are given within a set of brackets, followed by an abbreviation for the anion. Although not expressively written, it should be understood that the cation has a positive charge and the anion has a negative charge. For example, [BMIm]OAc indicates 1-butyl-3-methylimidazolium acetate, [AMIm]Cl indicates 1-allyl-3-methylimidazolium chloride, and [EMIm]OF indicates 1-ethyl-3-methylimidazolium formate.
Ionic liquids can be costly; thus, use of ionic liquids as solvents in many processes may not be feasible. Despite this, methods and apparatus for reforming and/or recycling ionic liquids have heretofore been insufficient. Furthermore, many processes for producing ionic liquids involve the use of halide and/or sulfur intermediates, or the use of metal oxide catalysts. Such processes can produce ionic liquids having high levels of residual metals, sulfur, and/or halides.